Cumulative sum in powerquery editor

Ok, I think I understand now. We will try the following:

• Group by DataDay and SUM the volume
  • Sort the table in Date order ascending
• Group by ExpiryDay and SUM the volume
  • Sort the table in Date order Ascending
• Add Running Total columns to each of the above Grouped tables
  • Using a custom function for this so as to simplify the code
• Correct the RT column of the DataDay table by subtracting the last amount in the ExpiryDate column where the Date in ExpiryDay RT is less than that in the DataDay RT

Note that the first lines are merely to transform your data so I can use it, since you only provided a screenshot and not text information that could be copy/pasted

Running Total Custom Function

Rename fnRT

//Running Total Returns a Table
//Rename: "fnRT"

(tbl as table, col as text)=> 


let 
    values = List.Buffer(Table.Column(tbl,col)),
    colNames= List.Buffer(Table.ColumnNames(tbl)),
    RT = List.Generate(
        ()=> [rt = values{0}, idx = 0],
        each [idx] < List.Count(values),
        each [rt = [rt] + values{[idx]+1}, idx = [idx]+1],
        each [rt]
    ),
    res = Table.FromColumns(
        Table.ToColumns(tbl) & {RT},
        colNames & {col & " Running Total"}
    )
in 
    res

Main Query

let
    Source = Excel.CurrentWorkbook(){[Name="Table3"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"DataDay", type date}, {"Volume", Int64.Type}, {"ExpiryDay", type date}}),

//Group Rows by DataDay; Sum the Volume
    #"Grouped DataDay" = Table.Group(#"Changed Type", {"DataDay"}, {
        {"Totals by DataDay", each List.Sum([Volume]), Int64.Type}}),
    #"Sorted DataDay" = Table.Sort(#"Grouped DataDay",{{"DataDay", Order.Ascending}}),

//Group Rows by Expiry Day; Sum the Volume
    #"Group ExpiryDay" = Table.Group(#"Changed Type",{"ExpiryDay"},{
        {"Totals by ExpiryDay", each List.Sum([Volume]), Int64.Type}
    }),
    #"Sorted ExpiryDay" = Table.Sort(#"Group ExpiryDay",{{"ExpiryDay", Order.Ascending}}),

//Add Running Total Column by DataDay
    #"RT DataDay" = fnRT(#"Sorted DataDay","Totals by DataDay"),
    #"RT ExpiryDay" = fnRT(#"Sorted ExpiryDay", "Totals by ExpiryDay"),

//Subtract ExpiryDays prior or equal to dataday
    #"Corr RT" = Table.AddColumn(#"RT DataDay", "Corrected Running Total", 
        (c)=> c[Totals by DataDay Running Total] - 
            (try Table.Last(Table.SelectRows(#"RT ExpiryDay", each [ExpiryDay] < 
                c[DataDay]))[Totals by ExpiryDay Running Total] otherwise 0), Int64.Type),
    #"Removed Columns" = Table.RemoveColumns(#"Corr RT",{"Totals by DataDay", "Totals by DataDay Running Total"})

in
    #"Removed Columns"

Data

Results